Lifeboats and Coastguard crews have been despatched to an area of the channel near Ramsgate after receiving reports the man fell in on Sunday morning.
The Coastguard said it was co-ordinating the search with the RLNI after the lifeboat service sent out crew from bases in Ramsgate and Dover. A helicopter is also scouring the area from the air.
The man had been onboard the AIDAperla cruiseliner, which had been travelling from Hamburg in Germany to Las Palma in the Canary Islands, according to data from the Cruise Mapper website.
The ship’s operator, AIDA Cruises, said the man who fell overboard was a member of the ship’s crew.
The company said in a statement: "The captain and crew of AIDAperla immediately initiated all necessary rescue measures in close coordination with the local authorities.
"The search on board confirmed that a male crew member is missing.
"The ship was immediately stopped and returned to the spot where the incident was believed to have taken part in the search."
In a statement, HM Coastguard said: "HM Coastguard is coordinating the search for a man reported overboard from a passenger ship off Ramsgate on 22 October. HM Coastguard was alerted at about 8.55am.
"The HM Coastguard search and rescue helicopter has been sent from Lydd alongside a Coastguard fixed-wing aircraft and the RNLI’s all-weather lifeboats from Ramsgate and Dover."
A spokesperson for the RLNI said: “Ramsgate RNLI’s all-weather lifeboat was paged at 08.20hrs this morning followed by Dover’s all-weather lifeboat at 08.35hrs. The search has been ongoing since then and an HM Coastguard helicopter has also been involved.
According to Cruise Mapper, the AIDAperla is an Italian-flagged vessel whose home ports are in Hamburg, La Romana in the Dominican Republic, Bridgetown in Barbados and Rotterdam in the Netherlands.
The website said the boat departed from Hamburg on 20 October and had been due to arrive in La Coruna, Spain, on Monday.
From there it was scheduled to travel on to Vigo, Spain, then to Lanzarote and Fuerteventura in the Canary Islands before arriving at its final destination of Las Palmas on 29 October.